Electronic converters for light sources comprising e.g. at least one LED (Light Emitting Diode) or other solid-state lighting means, may offer a direct current output. Such current may be steady or vary in time, e.g. in order to adjust the brightness emitted by the light source (so-called dimming function).
FIG. 1 shows a possible lighting arrangement comprising an electronic converter 10 and a lighting module 20 including, e.g., at least one LED L.
For instance, FIG. 2 shows an example of a lighting module 20 comprising e.g. a LED chain, i.e. a plurality of LEDs connected in series. As an example, FIG. 2 shows four LEDs L1, L2, L3 and L4.
Electronic converter 10 usually comprises a control circuit 102 and a power circuit 12 (e.g. an AC/DC or DC/DC switching supply) which receives at an input a power signal (e.g. from the mains) and provides at an output, via a power output 106, a direct current. Such a current may be steady or vary in time. E.g., control circuit 102 may set, via a reference channel Iref of power circuit 12, the current required by LED module 20.
For example, such a reference channel Iref may be used for adjusting the brightness of the light emitted by lighting module 20. As a matter of fact, in general terms, a regulation of the light brightness emitted by LED module 20 may be achieved by regulating the average current flowing through the lighting module, for example by setting a lower reference current Iref or by switching on or off power circuit 12 through a Pulse Width Modulation (PWM) signal, typically at a frequency of 100 to 500 Hz.
Generally speaking, there are many types of electronic converters, which are mainly divided into insulated and non-insulated converters. For example, among the non-insulated electronic converters we may name “buck”, “boost”, “buck-boost”, “Cuk”, “SEPIC” and “ZETA” converters. Insulated converters are e.g. “flyback”, “forward” converters.
For example, FIG. 3 shows a circuit diagram of a half-bridge converter 12 operating as a DC/DC converter. An input AC current may be converted into a direct current via a rectifier, e.g. a diode-bridge rectifier, and optionally a filter capacitor.
In the presently considered example, converter 12 receives at input, via two input terminals 110/GND1, a voltage Vin and provides at output, via two output terminals 106, a regulated voltage Vo or preferably a regulated current io.
In the presently considered example, a load R0 is connected with said output 106, and it may consist in the previously described lighting module 20.
Converter 12 moreover includes a half-bridge, i.e. two electronic switches S1 and S2 which are connected (typically directly) in series between both input terminals 110/GND1, wherein the switching of electronic switches S1 and S2 is driven by a control unit 112. For example, in the embodiment such electronic switches S1 and S2 are N-MOS transistors or n-channel MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). Such switches S1 and S2 may have respective capacitances CA1, CA2 and respective diodes DA1, DA2 connected therewith in parallel.
Typically, control unit 112 is configured for switching switches S1 and S2 alternatively, i.e. only one of both switches S1 and S2 is closed at a given time. Generally speaking, there may be also provided intermediate intervals during which neither switch S1 or S2 is closed. For this reason, control unit 112 is typically configured for driving switches S1 and S2 of half bridge S1/S2 with the following phases, which are repeated periodically:                during a first time interval Δt1 switch S1 is closed and switch S2 is opened;        during a second time interval Δt2 switch S1 is opened and switch S2 is opened;        during a third time interval Δt3 switch S1 is opened and switch S2 is closed;        during a fourth time interval Δt4 switch S1 is opened and switch S2 is opened.        
In the presently considered example, converter 12 moreover comprises a transformer T including a primary winding T1 and a secondary winding T2. Specifically, transformer T may be modelled as an ideal transformer having a given ratio of the number of turns 1:n, an inductor LM which represents the magnetising induction of transformer T, and an inductor LR which represents the leakage inductance, which are shown in FIG. 3 on the primary side of transformer T.
In the presently considered example, primary winding T1 of transformer T and at least one capacitor CRP are connected in series between the intermediate point between both switches S1 and S2 and the first input terminal 110 (positive terminal) and/or the second input terminal GND1 (a negative terminal representing a first ground). Specifically, in the presently considered example, the first terminal of primary winding T1 of transformer T is connected (e.g. directly) to the intermediate point between both electronic switches S1 and S2. On the other hand, the second terminal of primary winding T1 of transformer T is connected, via at least one capacitor CRP, to the first input terminal 110 and/or to ground GND. Therefore, switches S1 and S2 may be used for selectively connecting the first terminal of primary winding T1 of transformer T to voltage Vin or to ground GND1, thereby controlling the current flowing through primary winding T1 of transformer T.
On the secondary side T2 of transformer T, converter 12 comprises a rectifying circuit configured for converting the alternated current (AC) supplied by secondary winding T2 into a direct current (DC), and a filter circuit stabilizing the signal supplied by the rectifying circuit, so that output voltage Vo and/or output current io are more stable.
Specifically, in the presently considered example, the converter comprises, on the secondary side of transformer T, three branches which are connected in parallel, wherein:
a) the first branch comprises a first capacitor CRS connected in series with secondary winding T2 of transformer T,
b) a second branch, comprising a diode D, and
c) a third branch, comprising a second capacitor Co connected in series with an inductor Lo, wherein output 106 is connected in parallel with second capacitor Co.
For example, a first terminal of secondary winding T2 may be connected (e.g. directly) to the cathode of diode D, and the second terminal of secondary winding T2 is connected (e.g. directly) via capacitor CRS to the anode of diode D. Moreover, a first terminal of inductor Lo may be connected (e.g. directly) to the cathode of diode D, and the second terminal of inductor Lo may be connected (e.g. directly) via capacitor Co to the anode of diode D (which therefore represents a second ground GND2). The converter is asymmetrical because the on-times of S1 and S2 are typically different and mainly depend on the output voltage.
The converter shown in FIG. 3 offers the advantage that such a converter may be driven so that switches S1 and S2 are switched at zero voltage (Zero Voltage Switching, ZVS) and diode D is switched at zero current (Zero Current Switching, ZCS), achieving a so-called soft switching. Substantially, the ZVS and ZCS conditions may be achieved by correctly sizing the resonant components of the converter (i.e. inductances and capacitances).
Specifically, as previously stated, there are typically provided intermediate switching intervals, wherein neither switch S1 or S2 is closed. During such time intervals, the current on primary side of transformer T1 should charge and discharge capacitances CA1 and CA2 associated with switches S1 and S2, so that switches S1 and S2 may be closed in the following phase of zero voltage switching.
For example, details about the operation and the possible sizing of the circuit shown in FIG. 3 are described in document PCT/IB2014/064657, which is herein incorporated by reference in its entirety and for all purposes.
However, when the converter is used for driving a LED chain, or when the converter is used as a current generator (i.e. with current control), output voltage Vo depends on the number of LEDs L which are connected in series, i.e. voltage Vo should substantially correspond to the sum of the forward voltages of LEDs L. Therefore, the converter should be able to supply different output voltages, in order to support a variable number of LEDs. For example, the ratio between the minimum and maximum voltage capability is normally required to amount to three. Moreover, control unit 112 is often configured for regulating the supplied current io to a desired value, which may also be variable (see Iref in FIGS. 1 and 3), e.g. for implementing a dimming function.
Therefore, converter 12 should be sized so that it may handle various operating conditions, specifically as regards voltage and current variations.
However, this often implies that the converter is no longer able to guarantee a zero voltage switching of switches S1 and S2 and/or a zero current switching of diode D for all operating conditions.
Specifically, transformer T (magnetizing inductance LM and optional inductors connected in parallel with the primary and/or secondary windings) is typically sized in such a way as to ensure a correct ZVS for switches S1/S2 and a correct ZCS for diode D, and simultaneously to achieve a desired efficiency in all operating conditions.
In this case, current IP flowing through primary side T1 of transformer T charges and discharges capacitors CA1 and CA2, when neither switch S1 or S2 is closed, thereby creating the ZVS condition for switches S1 and S2.
The other components are sized in such a way as to ensure a correct resonance condition within the switching cycle.
In this situation, the inventors have observed that at low output voltages the ratio between the on-times of S1 to S2 reaches the lowest values. Specifically, the on-time of switch S2 must be sufficiently long as to subsequently enable the ZVS condition for closing switch S1, because the current flowing through switch S2 should become positive.
However, in this case the current flowing through diode D shows a higher oscillation. In this case, therefore, the opening of switch S2 may cause the loss of the ZCS condition on diode D.